This invention relates to a method and apparatus for automatic zero adjustment of a weighing machine for measuring the weighing of articles. More particularly, the invention relates to an automatic zero-adjustment method and apparatus for obtaining the difference between a zero-point correction value and an unloaded weight value (which is produced as an output by a weighing machine when the weighing machine is in an unloaded condition), for changing the zero-point correction value in such a manner that the difference takes on a set value, and for subtracting the difference from a weight value produced as an output by the weighing machine when the weighing machine is subjected to a load.
A weighing machine employed in a combinatorial weighing system or the like includes a weight sensor and a weighing hopper (or weighing dish), the former comprising a load cell. Articles introduced into the weighing hopper are weighed by the weight sensor which applies an output, indicative of the measured weight, is applied to a computerized combinatorial processing unit through an amplifier and an analog-to-digital converter (referred to as an AD converter). The combinatorial processing unit is operable to form combinations of weight values obtained from a plurality of the aforementioned weighing machine constituting the combinatorial weighing system, calculate the total weight of each combination, obtain a combination, referred to as an "optimum combination", whose total weight value is equal to a target weight value or closest to the target weight value within preset allowable limits, discharge the articles from the weighing hoppers of the weighing machines corresponding to the optimum combination, (whereby these weighing hoppers are left empty), resupply the emptied weighing hoppers of these weighing machines with articles in order to prepare for the next weighing cycle, and then repeat the foregoing steps in similar fashion to carry out a continuous automatic weighing operation.
The load cell and amplifier mentioned above have characteristics that vary with temperature. In addition, matter such as powder, oil and residue ascribable to the articles being weighed, affix themselves to the weighing hopper with time. Therefore, unless certain measures are taken, the tare weight signal output from the associated weighing machine tends to vary, thereby making it impossible to maintain good weighing precision. Accordingly, in the prior art, a zero-point adjustment circuit is provided for each and every weighing machine, and a zero-point adjustment is effected by the circuit to deal with a fluctuation in the tare weight value caused by a change in temperature and by the accumulation of residue or the like. However, since this conventional zero-point adjustment is performed by manual control of a variable resistor in the zero-point adjustment circuit, adjustment requires considerable time and effort, particularly when there are a large number of weighing machines.